If an impact is applied to a vehicle from a rear side in a state in which an occupant of the vehicle is not leaning against an upper body on a seat back, a head of the occupant rapidly moves to a head rest in accordance with a rapid movement of the upper body of the occupant to the seat back. At this time, a difference is generated between a moving amount of the upper body and a moving amount of the head of the occupant, and the head of the occupant moves rearward more largely than the upper body. Thereafter, the upper body of the occupant is largely oscillated forward on the basis of a reaction of the impact force. However, a load is applied to a neck region of the occupant on the basis of the difference of the moving amounts at this time.
Thus, conventionally, for example, as disclosed in Patent Documents 1 and 2, there has been proposed a head rest device provided with a mechanism of moving the head rest forward to the head of the occupant at a time when an impact is applied to the vehicle from the rear side. On the basis of the structure described above, the head of the occupant is prevented from moving backward largely at a time when the shock is applied from the rear side, and the load applied to the neck region of the occupant is reduced.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-13604
Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-87650